Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil and atmospheric residue. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
To address coking problems, U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a pre-determined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
In using a flash to separate heavy liquid hydrocarbon fractions from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components depart the separator in the liquid bottoms stream. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems. However, the flashing in a flash/separation vessel (also referred to herein as a “vapor/liquid separator”) is typically accompanied by coking of internal surfaces in and proximally downstream of the vessel. The extent of such coking is dependent upon various factors including feed type, preheating protocol, and design of the vessel. Heavy hydrocarbon liquids contacting the internal surfaces of the vessel and downstream equipment provide coatings of films that are precursors to coke. Excessive temperatures in the separator vessel and downstream vapor piping, such as above about 427° C. (800° F.), typically from about 450 to about 460° C. (840 to about 860° F.) or from about 510 to above about 621° C. (950 to 1150° F.), depending on the feedstock, are theorized to lead to excessive coke formation by thermal cracking and heat soaking of the heavy end of the heavy hydrocarbon feedstock stream. Because this coke buildup can effect restriction and increase pressure drop within the overall process, it would be advantageous to control the coke buildup within the flash zone and immediately downstream of the flash zone.
U.S. Pat. No. 7,244,871, which is incorporated herein by reference, discloses a process for removing coke formed during cracking of hydrocarbon feedstock containing resid and coke precursors, wherein steam is added to the feedstock to form a mixture which is thereafter separated into a vapor phase and a liquid phase by flashing in a flash/separation vessel. The vapor phase is then separated and cracked and the resulting cracked product recovered. Coking of internal surfaces in and proximally downstream of the vessel is controlled by interrupting the feed flow, purging the vessel with steam, introducing an air/steam mixture to at least partially combust the coke, and resuming the feed flow when sufficient coke has been removed.
Controlling the ratio of vapor to liquid leaving the flash has been found to be difficult because many variables are involved, including the temperature of the stream entering the flash, which varies as the furnace load changes. The temperature is higher when the furnace is at full load and is lower when the furnace is at partial load. The temperature of the stream entering the flash also varies according to the flue-gas temperature in the furnace that heats the feedstock. The flue-gas temperature in turn varies according to the extent of coking that has occurred in the furnace. When the furnace is clean or very lightly coked, the flue-gas temperature is lower than when the furnace is heavily coked. The flue-gas temperature is also a function of the combustion control exercised on the burners of the furnace. When the furnace is operated with low levels of excess oxygen in the flue gas, the flue-gas temperature in the middle to upper zones of the convection section will be lower than that when the furnace is operated with higher levels of excess oxygen in the flue gas.
U.S. Pat. No. 7,138,047, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water. To avoid coke deposition in the first stage of preheating in the convection section (and excessive coking in the radiant and quench sections) the mixed and partially vaporized feed and dilution steam stream is generally withdrawn from the convection section before the feed is fully vaporized and before excessive film temperatures are developed in the convection section tubes. Excessive film temperatures, such as above about 510° C. (950° F.) to above about 620° C. (1150° F.) depending on the feedstock, are theorized to lead to excessive coke formation from the heavy end of the heavy hydrocarbon feedstock stream.
Further efforts to reduce coking after the flash are disclosed in U.S. Published Patent Application No. 2005/0209495, which is incorporated herein by reference, which discloses a process for cracking heavy hydrocarbon comprising heating the heavy hydrocarbon feedstock, mixing the heavy hydrocarbon feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase, separating and cracking the vapor phase, and cooling the product effluent in a transfer line exchanger, wherein the amount of the fluid and/or the primary dilution steam stream mixed with the heavy hydrocarbon feedstock is varied in accordance with at least one selected operating parameter of the process, such as the temperature of the flash stream before entering the flash/liquid separator vessel.
However, even in light of the aforementioned advances, coking of the various internal contact surfaces of a pyrolysis furnace remains a problem. In the past, removal of coke from one or more of the furnace sections required interrupting the normal production schedule of the furnace to conduct the decoking process. Such interruptions can cause serious economic problems during the period of time that the furnace is off-stream.
Efforts to address this problem are disclosed in U.S. Pat. No. 3,365,387, which is incorporated herein by reference, which discloses a process of decoking of a minor portion of the thermal cracking tubes in a furnace while the major portion of the thermal cracking tubes remain in service processing hydrocarbon feed and producing olefin products, conventionally known as “on-stream decoking”. A further improvement of the on-stream decoking process is disclosed in U.S. patent application Ser. No. 12/172,048, filed Jul. 11, 2008, which is incorporated herein by reference, which is directed to on-stream decoking of a more modern pyrolysis furnace design, containing multiple banks of tubing within the convection section of the furnace and a vapor/liquid separator vessel upstream of the radiant section of the furnace.
However, prior decoking methods either resulted in sending coke residues into decoking drums and venting gases such as CO and CO2 into the atmosphere, or in the case of on-stream decoking, sending coke residues further downstream through the system to be removed later in the process. In either case, it was necessary to collect the coke residues and send it to off-site waste disposal.